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Name: John
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Question:
If you are holding a candle in an elevator on the 11th floor(a lit candle) & the cable breaks, does the candle remain lit as you plummet to the bottom? Why or Why not?



Replies:
John,

Thanks for your question.
My answer would be that since the elevator is a semi-sealed system, we assume there would be no major drafts as you plummet to your doom. Assuming no major drafts, the candle would continue burning (at least until the paramedics arrive). Seriously, you can ask you folks about this. If they drive an automobile and stick a match to light a cigarette, even though the semi-sealed system (car) is moving (here horizontally instead of vertically) the match would remain lit until it either runs out of fuel (end of match stick, or runs out of oxygen). The other way the flame could be extinguished is to open a window and allow the semi-sealed system to be opened. Naturally a strong draft would extinguish the flame, as the system is not sealed and the lit match experiences actual movement past a large volume of air (something we call wind). You can mimic the wind by blowing a large volume of air past the match also. A question for you: when you 'blow out' a candle, what exactly are you doing? How is the flame extinguished? How about when you extinguish a flame with water? Describe why the flame is no longer burning. Please send you replies here. Thanks for using Newton!

rickru


Well, I have heard about doing this experiment with a candle and a large (draft free) enclosed container. The result is supposed to be that the candle goes out almost immediately and the rational I have heard makes sense. The explanation is goes back to Galileo's discovery that similar objects of different masses fall at the same rate. In your enclosed elevator t there are three objects of importance and they all fall at the same rate. They are: the candle (and flame); the "normal" or unheated/unburned air; and the heated or burned air (oxygen depleted). Normally the heated air would rise, since it is _LIGHTER_ (or less dense) than the cool air. As it rises it is replaced with fresh air with a supply of oxygen to sustain the flame. In the free-falling, closed container everything falls at the same rate -- i.e., there is NO RELATIVE motion of the flame, the cool air and the oxygen depleted hot air. The result is that the oxygen in the vicinity of the flame is rapidly depleted and the flame dies due to oxygen starvation. Now that we have two opposing views it is time for peer review.

Greg
gregory r bradburn


Greg,

I believe your assertion that similar objects of different masses fall at the same rates is true in a vacuum, but clearly a candle could not burn in a vacuum. rickru


Wow! I am with Dr. Blackburn's beautiful reply. Fires clearly depend on the flow of combustion reactants into and products out of the reaction zone. This flow also clearly depends normally on the rising of air as it is heated (try lighting a fire in the fireplace with the chimney damper closed). But hot air only rises relative to cold air when it *weighs* less, and in the elevator in free fall everything is weightless. Of the varying terminal velocities of candle, air, and elevator in atmosphere only the last is relevant, since all objects are enclosed in the elevator. When the terminal velocity of the elevator (at a guess several hundred miles an hour) is reached weight will return and the candle can burn efficiently again. You could try the experiment in the Space Shuttle, but I would not think they would allow open flames.

christopher grayce


All objects will fall at the same rate whenever the net forces acting on them are the same. When not in a vacuum, i.e., falling through the air, there are viscous effects which affect the rates. However, this problem specifies a closed container so the viscous effects act only on the container. I assumed that the mass of the container is large enough and its shape is such that its terminal velocity is large -- actually I had not considered terminal velocity at all until Chris mentioned it. Of course I had also not considered trying to light a candle on the space shuttle -- the problems you run into with space travel!

gregory r bradburn


I will have to cast my vote with Chris Grayce. See my simple response to #130 in the Physics Archive Section.



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